1. Field of the Invention
The present invention relates to a semiconductor device, and, particularly, to a MOS transistor connected to an external output terminal.
2. Description of the Related Art
It is well known that when an electrified human body or device comes in contact with an IC terminal, the IC breaks down because of ESD (Electro Static Discharge).
As a countermeasure against the ESD, a protection element is normally formed in the inside of the IC. FIG. 3 shows a schematic view showing an output circuit provided with a protection element. In FIG. 3, a transistor 108 operating as an IC function is connected in parallel with a transistor 109 functioning as the protection element. Since a gate electrode 102b is connected to Vss, the protection transistor 109 can not operate. That is, the protection transistor 109 is in a stopped state.
Drains 105a and 105b of the respective transistors are connected to a power source voltage terminal Vdd and an output terminal 107, respectively, and sources 106a and 106b are connected to a Vss terminal (=GND). Normally, with respect to the surface breakdown of the respective transistors, that of the stopped transistor 109 is made lower than that of the operating transistor 108.
When this structure is adopted, even if static electricity enters from the outside, the stopped protection transistor 109 first breaks down to release the electric charge, so that the operating transistor 108 can be protected.
FIG. 2 is a layout view of a conventional semiconductor device. Normally, an operating transistor 108 is provided with two gate electrodes 102a at both sides of a drain 105a. In this case, a junction of the drain 105a and a field doped region (outside of a region 101a) is formed. A field concentration is set so that a parasitic channel is not formed on the surface of a silicon substrate under a field oxide film. The junction withstand voltage of the drain 105a and the field is changed by each concentration, and as the concentration becomes high, the withstand voltage becomes low.
Normally, the concentration is set so that the junction withstand voltage becomes higher than the surface breakdown of the transistor 108. However, according to a process, there is a case where the field concentration becomes high, so that the junction withstand voltage becomes lower than the surf ace breakdown. When the junction withstand voltage becomes lower than the surface breakdown of a transistor 109 connected in parallel, or the junction withstand voltage of a drain 105b of the transistor 109 and a field doped region (outside of a region 101b), the ESD comes to be applied to the transistor 108 and breaks down the transistor 108.
Like this, in the case where the junction withstand voltage of the drain of the operating transistor and the field doped region is lower than the surface breakdown of the operating transistor, and the junction withstand voltage becomes lower than the surface breakdown of the stopped transistor, there has been a possibility that the ESD is applied to the operating transistor and the element breaks down.
The present invention has been made in view of the above drawbacks in the conventional art and has an object to provide a semiconductor device which can solve the above problem.
If the withstand voltage of the transistor 108 is higher than the withstand voltage of the transistor 109, the ESD is applied to the transistor 109 as a protection element, and the transistor 108 can be protected.
Accordingly, the present invention adopts the following means.
(1) In a circuit in which an operating MOS transistor and a stopped state MOS transistor are connected in parallel with each other, a drain of the operating MOS transistor is surrounded by a gate electrode.
(2) The operating MOS transistor is connected to an external output terminal.
(3) A gate electrode of the stopped state MOS transistor is connected to GND.
(4) The circuit in which the operating MOS transistor and the stopped state MOS transistor are connected in parallel with each other is made a part of an output circuit of a voltage detection IC.
(5) The circuit in which the operating MOS transistor and the stopped state MOS transistor are connected in parallel with each other is made a part of an output circuit of a battery protection IC.
(6) A source/drain of the operating MOS transistor is made a concentration region of more than or equal to two kinds of low concentration and high concentration.
(7) A source/drain of the stopped state MOS transistor is made a concentration region of one kind of high concentration.
(8) The source/drain of the stopped state MOS transistor is made of phosphorus.
(9) The operating MOS transistor is made such a transistor whose a junction withstand voltage is higher than a surface breakdown.
(10) The surface breakdown of the stopped state MOS transistor is made lower than the surface breakdown of the operating MOS transistor.
(11) The maximum operating voltage of the voltage detection IC is made 7 V to 12 V.
(12) The maximum operating voltage of the battery protection IC is made 7 V to 12 V.